1. Field of the Invention
The invention present invention relates to novel, hydrophilic-rich organomethylsiloxane-dimethylsiloxane-polyoxyalkylene copolymers and to their use as emulsifiers for the preparation of cosmetic emulsions of improved aesthetic properties and mildness.
2. Background Art
An emulsion is a dispersion of droplets of one liquid in a second, immiscible liquid.
The process of dispersing one liquid in a second immiscible liquid is called emulsification.
An emulsifier is required to stabilize an emulsion. Emulsifiers are surface-active agents, that is a soluble compound that reduces the surface tension of liquids, or reduces the interfacial tension between two liquids. Surface tension is the force acting on the surface of a liquid, tending to minimize the area of the surface. Surfactants and emulsifiers (all emulsifiers are a class of surfactants) reduce the surface tension of a liquid. The surface tension of water is 72 dyne/cm; a surfactant can reduce this to a value in the range of 30-50 dyne/cm.
There are two basic types of emulsions: oil in water (o/w) and water in oil (w/o). The substance named last indicates the continuous or outer phase. Whether a particular oil and water blend will form a w/o or o/w emulsion depends largely on the choice of emulsifier. Emulsifiers that are more soluble in water than in the oil generally produce o/w emulsions. Emulsifiers that are more soluble in the oil produce w/o emulsions.
Choice of emulsifier type is dependant upon the final skin feel desired. The continuous phase will be the most perceived phase while applying the lotion and after dry down. Water in oil emulsions (w/o) tend to be heavier, leave a more distinctive after-feel and are considered warmer than emulsions with water as the continuous phase (o/w). When water is the continuous or outer phase, typically the formulation is lighter, easier to rub in to the skin, and the after-feel is lighter. The overall skin conditioning efficacy of both emulsion types o/w and w/o is dependant on the relative concentrations of the ingredients used.
A predominantly hydrophilic emulsifier, will best stabilize o/w emulsions, while a predominantly lipophilic emulsifier, will best stabilize w/o emulsions.
HLB, the hydrophile/lipophile balance of a surfactant, can help determine the best surfactants to use as emulsifiers. As expected, products with higher ethylene oxide content are more suitable for o/w emulsification, while products with lower ethylene oxide content are more suitable for w/o emulsification.
A review of predominant skin care products currently in the market shows strong regional preferences for specific emulsion types. Typically European markets sell heavier, more occlusive formulations and as such have a strong preference for w/o formulations. Outside of the European markets, the overwhelming global preference of lotions is o/w. This preference is most predominant in Asian markets. The o/w formulations provide excellent skin treatment products without leaving excessively coated heavy feeling. This is most preferable in hot humid environments. Very few if any w/o formulations are even sold in Asia. The United States and Latin American markets also have strong preferences toward the lighter feeling o/w formulation, however there are some brands sold using w/o technology.
The alkyl dimethicone copolyol emulsifiers of this invention are unique in that they serve as o/w emulsifiers and provide stable o/w emulsions. This is in contrast to alkyl dimethicone copolyol emulsifiers of the prior art (see, for example, U.S. Pat. No. 4,698,178), which specifically serve as w/o emulsifiers and provide stable w/o emulsions. The alkyl dimethicone copolyol emulsifiers of this invention have an additional benefit of providing lift from the surface, creating an overall skin feel that is soft and extremely smooth. This softening effect is important beyond aesthetics in that it reduces the overall roughness of the surface of the skin, allowing light to reflect more evenly. The skin appears more uniform in color and texture, a benefit strongly desired in anti-aging skin care products. This benefit is not observed using either the aforementioned alkyl dimethicone copolyol w/o emulsifiers of the prior art (such as Goldschmidt ABIL EM90) or o/w emulsifiers of the dimethicone copolyol variety (such as Dow Corning DC 193) or o/w emulsifiers of the non-silicone containing variety (such as Glyceryl Stearate and PEG 100 Stearate). Additional benefits provided by the emulsifiers of this invention over the aforementioned emulsifiers are a reduction of greasy feeling for emulsions that have high (15% or higher) oil and/or petrolatum content, a brilliant white appearance and less dense, soufflé-like texture for said high oil and or petrolatum emulsions, a decreased rub in time, a final rub out that is less greasy, and a markedly decreased irritancy.
There is considerable prior art relating to polydimethylsiloxane-polyoxyalkylene copolymers (silicone copolyols) and organomethylsiloxane-dimethylsiloxane-polyoxyalkylene copolymers (organo-functional silicone copolyols).
Organo-functional silicone copolyols related to those of this invention have been prepared from silicon hydride containing siloxanes of the general structureMe3SiO(Me2SiO)x(HMeSiO)ySiMe3 wherein Me is methyl, x is 0 to about 200, and y is about 1 to 100, via a hydrosilation coupling reaction (that utilizes a platinum catalyst) with terminally unsaturated polyoxyalkylenes and terminally unsaturated organic molecules. The synthesis of organo-functional silicone copolyols is described in U.S. Pat. Nos. 3,234,252, 4,047,958, 3,427,271, 2,846,458, and 6,346,553, and yields products of the general formulasMe3SiO(Me2SiO)x(MeRSiO)y(MeQSiO)zMQMe2SiO(Me2SiO)x(MeRSiO)ySiOMeQwherein R denotes an alkyl radical and Q denotes a polyoxyalkylene radical having the formula —CH2CH2CH2(OCH2CH2)p(OCH2CHCH3)qOR′ wherein R′═H or Me and both p and q range between 0-50. These preparations are similar to those for the preparation of silicone copolyols. The preparation of such copolymers by the platinum catalyzed hydrosilation of silicon hydride containing siloxanes and a polyoxyalkylene that is allyl terminated at one end and either OMe or O(CO)R terminated at the other end is well known and straightforward. The similar preparation of silicone copolyols that employ polyoxyalkylene but that is allyl terminated at one end and OH terminated at the other end is more problematic. In all reported cases prior to this invention, steps are taken in an attempt to eliminate or minimize competing crosslinking side reactions that involve the polyoxyalkylene terminal OH function and are well know to those practiced in the art. One such crosslinking side reaction is due to reaction of the Si—H and COH groups. Another is conversion of terminal allyl groups to propenyl groups followed by crosslinking via acetal formation with a polyoxyalkylene OH terminus. These crosslinking reactions are catalyzed by platinum catalysts such as chloroplatininc acid (H2PtCl6.6H2O), and lead to a generally undesired increase in viscosity of the final product and/or eventual gellation.
Much of the prior art relating to the preparation of silicone copolyols, embodied in U.S. Pat. Nos. 3,280,160, 3,401,192, 4,122,029, 3,518,288, 4,520,160, 5,958,448, 6,346,583, and 6,346,595, discloses the use of one or more solvents, often a small cyclic polydimethylsiloxane or an alcohol, to minimize the aforementioned side reactions. A solventless process is disclosed in U.S. Pat. Nos. 4,847,398 and 6,372,874 that employ catalyst modifiers which also minimize the aforementioned crosslinking side reactions. In U.S. Pat. No. 4,025,456, a solventless process for the preparation of siloxane-polyoxyalkylene copolymers is revealed. This reference, however, utilizes alkoxy endblocked polyethers in the hydrosilation reaction and does not disclose the use of OH terminal oxyalkylene polyethers.
Similarly, much of the prior art relating to the preparation of organo-functional silicone copolyols discloses the use of one or more solvents or endblocked (protected) polyethers to minimize the aforementioned side reactions. In U.S. Pat. No. 6,346,553, a solventless process for the preparation of organo-functional silicone copolyols is revealed that utilizes alkoxy endbolcked polyethers. Another common method is to protect the polyoxyalkylene OH termini via conversion to OSiMe3, followed by deprotection after hydrosilation, as is the case in U.S. Pat. No. 2,846,458.
Silicone copolyols and organo-functional silicone copolyols are commonly used as emulsifiers for the preparation of cosmetic formulations, a fact well known to those practiced in the art. Typical examples of silicone copolyols are Me3SiO(Me2SiO)22(MeQSiO)4SiMe3 and Me3SiO(Me2SiO)8.7(MeQSiO)3.7SiMe3 (where Q=(CH2)3(OCH2CH2)12OH). These and similar materials are “water soluble” (i.e., they are either truly soluble or they form stable dispersions in water). According to U.S. Pat. Nos. 4,381,241, and 4,698,178, rake-type organo-functional silicone copolyols of the type Me3SiO(Me2SiO)x(MeRSiO)y(MeQSiO)zM can also be used as emulsifiers for the preparation of cosmetic emulsions. The ABA or end-blocked organo-functional silicone copolymers QMe2SiO(Me2SiO)x(MeRSiO)ySiOMeQ of U.S. Pat. No. 6,346,553 can be used as emulsifiers for the preparation of combined oil-silicone O/W emulsions. In all of these cases, the preferred formula is one in which the number of units having alkyl (R) radicals is at least twice as large (and typically much greater than twice as large) as the number of units with polyoxyalkylene (Q) radicals. This preference, along with the preferred length of the alkyl radical and the preferred composition of the polyoxyalkylene radical (which generally includes water insoluble propylene oxide units along with water soluble ethylene oxide units) results in predominantly hydrophobic (water insoluble) organo-functional silicone copolyols. One such example, a rake-type alkylmethylsiloxane-dimethylsiloxane-polyoxyalkylene emulsifier of the type described in U.S. Pat. No. 4,698,178, is available under the name Goldschmidt ABIL EM90 and is insoluble in water. Thus, whereas organo-functional silicone copolyols of the prior art are insoluble in water, the organo-functional silicone-copolyol emulsifiers of this invention are soluble in that they form stable dispersions in water at ≧15% mass emulsifier.
Many modern cosmetic formulations that make use of emulsifiers of the type described above and/or other emulsifiers have a high oil content (>20% by mass oils). Desirable aesthetic effects for such cosmetic formulations include a light texture, an initial rub-in that possesses a light, slightly wet, cool feel (as opposed to a heavy, greasy, warm feel), a short overall rub-in time, and a final rub-out that is not heavy, greasy, or tacky, but instead leaves a light occlusive barrier on the skin. Also desirable is a minimization of skin irritancy (i.e., maximum mildness). Emulsifiers that can bestow these properties to cosmetic formulations would be desirable and valuable.